Inductive apparatus



1955 J. H. CHILES, JR., EI'AL 2,725,502

INDUCTIVE APPARATUS 2 Sheets-Sheet 1 Filed May 17, 1952 Fig. l.

Volts F lg 3 INVENTORS John H. Chil Jr. and %3 wcmE 200 F lg 2WITNESSES:

Nov. 29, 1955 J. H. CHILES, JR., ETAL INDUCTIVE APPARATUS 2 Sheets-Sheet2 Filed May 17, 1952 Volts Fig.5

Volts 78 Fig.7

Fig.6

INVENTORS John H. Chiles qr. and

WITNESSES:

United States Pate 2,725 502 INDUCTIVE APPARATUS- John H. Chiles, Jr.,and Albert I. Maslin, Sharon, Pa, assignors to Westinghouse ElectricCorporation, East Pittsburgh, Pa., a corporation rah-PennsylvaniaApplication May 17, 1952, Serial No. -288,484 4 Claims. (Cl. 317- 14) Anobject of this invention is to provideforminimizing any increase in thecore loss of inductive apparatus that occurs when one of its windings issubjectedto: anE impulse voltage, by preventing the establishmentofraconducting path that encloses a large cross-sectional areaofthe'laminated core to thuslimit the electromagneticvoltage that canact along the path that maybe established in the core, and by groundingthe core at a number of predetermined points whereby the magnitude anddistribution of the electrostatic voltage induced inthe-:.corei-is.chan-ged.

Another object of this invention is:to-provide:for-mini' mizing anyincrease in the core'loss.of'theztransformer that occurs whenone of itswindingszis-subjected to an impulse voltage, by dividing the insulated:transformer core laminations into sections and separatingthe sectionsby material having a predetermined dielectric-strength to therebyprevent the establishment of aconducting path that encloses a largecross-sectional area ofthedam'inated' transformer core, and bygroun-dingteachmf the sectionsat a predetermined pointwherebythevmagnitude-and distribution of the electrostatic voltage induced inthe core is changed.

Other objects of this invention WiIl Fbecome-apparentfrom the followingdescription whenita keirin conjunction with the accompanyingdrawings',-in' Which-2 Figure 1 is a schematic illustrationof atransformer illustrating one embodiment of thisinv'ent'i'on;

Fig. 2 is a schematic illustration of a transformer core constructed inaccordance-with the prior art;

Fig. 3 is a graph illustrating: voltage gradients for the transformercore of Fig. 2, when .jdifferent' factors are taken into consideration;-

Fig. 4 is a fragmentaryview of th'e'transformer core shown in Fig. l andwhich illustrates one -embodiment of the teachings of this invention.

Fig. 5 isa graph illustrating various-voltage gradients for thetransformer core of Fig. 4wh'e'n differentfac'tors are taken intoconsideration; j I

Fig; 6 is a fragmentary view o'fa transformercoreillus tratin'g anotherembodiment of this invention;

Fig. 7;is a graph illustrating various voltage, gradients for, thetransformer core .of Fig.;6 Wlienjdifferent factors are taken intoconsideration;

Fig. 8' is a fragmentaryview' ofa transformer core illustrating stillanother embodiment of this invention, and

Fig. 9'isa graph illustrating various'vol'tage gradients for inxaconventional manner.

v ance of the core 30 is considered, the

2,725,502 Patented Nov. 29, 1955 2 the transformer core of Fig. 8 whendifferent factors are taken into'consideration.

Referring to Fig. 1 of the drawings, there is depicted a transformer 10illustrating an embodiment constructed in accordance with the teachingsof this invention. In this instance, the transformer 16) comprises ashell-type magnetic core 12;,however, it is to be understood that thisin vention is equally applicable to other types of transformer cores.

' As illustrated, a primary winding 14 and a secondary winding 16 arewound in a conventional manner around the transformer winding leg 18 andare disposed in inductive relationship to the core 12, therebysubjecting the core 12 to an electrostatic and electromagnetic fieldwhen either of the transformer windings id or 16 is subjected to animpulse voltage.

Since this invention concerns the manner of constructing the transformercore 12 and grounding the same, a complete showing of a transformer case20 and a showing of the conventional bushings disposed therein forproperly insulating the primary and secondary lead-in connections isdeemed unnecessary.

As illustrated, the core 12 comprises a predetermined number of.laminations 22 made from magnetic sheet material, each lamination beingprovided on both sides with an adherent insulating layer or film ofdielectric material The laminations 22 in turn are separated into twocore sections 24 and 26 of substantially equal size by a member 28 ofpredetermined dielectric strength, the purpose. of which will bedescribed hereinafter. 'As illustrated, the member 28 will be so shapedand of such a size relative to the laminations 22' as togive adequateseparation and insulation as determined in" the design. In a particularapplication, the size of the core sections and thus the number ofseparated core sections required to provide a predetermined corestructurewill depend on the magnitude of the electromagnetic fieldproduced'by the current flowing through either the primary winding 14 orthe secondary winding 16 due to the impulse voltage, the" size beingpredetermined to resist arcing between the adjacent core laminations'22'. Each of the end lamination's 22' disposed on opposite sides of thecore'lZ is grounded, the purpose of which will likewise be describedhereinafter.

Referring to- Fig. 2, there is illustrated a section of a conventionallaminated transformer core 30 in which one of the end laminations isgrounded. The transformer core 30 comprises a plurality of laminations32 made from magnetic sheet material which are substantially insulatedfrom one another in a conventional manner by means of suitabledielectric material 34. One side of the core 30 is grounded by a groundconnection 35.

Referring to Fig. 3, there is illustrated a graph representing:electrostatic voltage gradients for the transformer core 30 whendifferent factors are taken into consideration. In particular, a curve36 represents the electrostatic voltage gradient for the core 30 whenonly the capacitance between adjacent laminations 32 of the core 34 andthe capacitance established by the capacitive coupling between thewindings (not shown) and the core 39 is considered. However, if inaddition to' these capacitances the resistcurve 38 representsthezvoltagegradient. When an impulse voltage is applied to eithertheprimary winding (not shown) or thesecondary winding (not shown), thetransformer winding is increased in potential far above zero or groundpotential.

.However, the portions of the core 30 remote from the mum at the pointclosest to the windings and farthest from the ground connection 35, andwill decrease, as illustrated in Fig. 3, as the ground connection 35 isapproached. In some cases, this electrostatic potential gradient alongthe core 3% is sufficiently high to cause the electrical breakdown ofthe dielectric material 34 between the individual laminations 32. Whenthis takes place, the resistance of the core 30, as measured from top tobottom, is greatly reduced. In spite of this breakdown and greatlyreduced resistance of the built-up core 30, however, an increase in coreloss does not necessarily result. This is because the conducting pathcreated by the breakdown may not be a closed path encircling all or partof the core crosssection. If the path, however, encircles all or part ofthe core cross-section, the presence of normal frequency flux in thecore 31 will create a current around this path, thus resulting in anincrease in core loss.

The above-discussed effect of electrostatic voltage induced in the core39 is not the only effect that takes place when an impulse voltage isapplied to either of the transformer windings (not shown). When theimpulse voltage is applied, it distributes itself along the electricallength of the transformer winding (not shown), and effects a largecurrent flow through the winding, which in turn, gives rise to a flux inthe core 36 which follows the normal flux path of that core. This fluxin turn will induce an electromagnetic voltage in any closed pathsurrounding all or part of it. Such a path may be wholly within thecross-section of the core 30 or it may correspond wholly or in part withthe periphery of the crosssection of the core 30. The electromagneticvoltage induced in such a path by the flux in the core may be suflicientto break down the insulation 34 or jump the minute gaps at the peripherywhere there are burrs on the individual laminations 32, and thus permitthe flow of current around the path, thus resulting in an increase incore loss. It is believed that both the electrostatic voltage due to thepresence of the core 30 in the electrostatic field, and theelectromagnetic voltage due to the electromagnetic field, cooperate inthe creation of closed conducting circuits within, or surrounding thecore 30, which give rise to the increase in core less experienced wheneither of the transformer windings (not shown) is subjected to animpulse voltage. It is further believed that the electrostatic voltageon the core 30 may effect a breakdown of the dielectric material 34 eventhough a closed electrical circuit is not established by such an action.Then when the electromagnetic voltage is induced in the core 30, it willcomplete the path or a part of the path originally established by theelectrostatic voltage so as to form a completed electrical circuitwithin the core 30 and thus increase its core loss.

Referring to Fig. 4, the transformer core laminations 22 of Fig. 1 areseparated into the two core sections 24 and 26 by the dielectric member28, and each end lamination of the laminations 22 disposed on oppositeends of the core 12 is grounded. The laminations 22 in turn areinsulated from one another by means of dielectric material 42 formed bythe adherent insulating layers disposed on both sides of each of thelaminations 22. By subdividing the core 12 by means of the dielectricmember 28, the occurrence of a conducting path enclosing a largecross-sectional area of the core 12 is prevented. Thus theelectromagnetic voltage which can act along any path that may beestablished within the core sections 24 and 26 is limited. Inparticular, if the transformer core is divided into N sections, thevoltage induced electromagnetically in any closed circuit involving thecore structure will be reduced by the factor when compared with thevoltage induced electromagnetically in a core not so subdivided. Inpractice, it has been found that when the dielectric member 28 has athickness of between /1 and of an inch, depending upon the rating of thetransformer, the thickness of the member 28 is suflicient to prevent anelectrical breakdown thereacross. However, it has been found byinvestigation that the thickness of the dielectric member 23 may bereduced to an insulating value substantially equal to the insulatingvalue of the sum of the dielectric material 4-2 provided in either ofthe core sections 24 or 26. For instance, the laminations 22 disposed onopposite sides of the dielectric material 28 may be bent towards oneanother at their edges during the course of manufacturing the core 12,thus limiting the effectiveness of the dielectric material 28.Therefore, in practice, it has been found desirable to increase thethickness of the dielectric material 28 to offset such an effectproduced by the bending of the edges of the laminations 22 disposed onopposite sides of the material 28. However, the thickness of thedielectric material 28 can be properly determined by one skilled in thetransformer art taking into consideration all the problems involved.

In practice, it has been found that the dielectric material may beformed from pressboard, varnished cloth, resin bonded laminate, or anyrecognized insulating material having relatively high compressivestrength.

In order to change the magnitude and distribution of the electrostaticvoltage induced in the core 32 when either the primary winding 14 or thesecondary winding 16 is subjected to a surge voltage, each of the endlaminations 22 disposed on opposite sides of the core 12 is grounded bythe ground connections 44 and 46, respectively. Thus, by grounding thecore 12 as illustrated in Fig. 4, the magnitude of the electrostaticvoltage appearing across either the core section 24 or 26 is reduced tosubstantially /2 the magnitude of that voltage appearing across the core30 illustrated in Fig. 2 for the same given set of conditions. This canbe more clearly seen by reference to curve 48 of Fig. 5 which representsa distribution of the electrostatic voltage across the core 12 whentaking into consideration only the capacitance established betweenadjacent laminations 22 and the capacitance established by thecapacitive coupling between the windings 14 and 16 and the core 12. Onthe other hand, curve 5 represents the distribution of voltage on thecore 12 when in addition to these capacitances, the resistance of thecore 12 is taken into consideration. It is believed that the actualvoltage gradient for the core 12 lies somewhere between the curves 48and 50 and it is illustrated by the curve 52. From the curve 52, it canbe seen that there is a considerable voltage drop across the dielectricmaterial 28. Due to this voltage drop, the dilference in voltage betweenadjacent laminations 22 in the core section 26 is reduced, thusaffording further protection against the formation of conducting pathswithin the core section 26, which effect an increase in the core loss.

Referring to Fig. 6, there is illustrated another embodiment of thisinvention, in which for the purpose of simplifying the drawings, only amagnetic core 54 is illustrated. However, it is to be understood thatthe core 54 has disposed in inductive relationship therewithwindingssimilar to the primary winding 14 and secondary winding 16illustrated in Fig. l and the core 54 is likewise disposed in atransformer case (not shown). The magnetic core 54, made from magneticsheet material, comprises a predetermined number of laminations 55, andit is divided into core sections 56, 58 and 60 by means of sheets ofmaterial 62 and 64 of predetermined dielectric strength. As illustrated,the core section 58 is approximately twice the size of either the coresection 56 or the core section 60. The sheets of material 62 and 64 aremade from the same material discussed with reference to the dielectricmaterial 28 illustrated in Fig. 4. In practice, the thickness of thesheets of material 62 and 64 is also the same as the thickness of thedielectric material 28 illustrated in Fig. 4, and the thickness isdetermined by the same factors discussed with reference to thedielectric material 28.

The sheets of dielectric material 62 and 64 are disposed in the core 54in order to prevent the establishment of a conducting path. thatencloses a large cross-sectional area of 'the"core"54', and thus preventthe establishment of a large induced electromagnetic voltage across thelaminations 55, which have'disposed therebetween a suitable dielectricmaterial 66.v In particular, the magnitude of the electromagneticvoltage induced in the core sections 56 and 60, comparedto the magnitudeof the electromagneticvoltage induced in the core 38 of 'Fig. 2 for thesame given set. of conditions is only one-fourth as great, while themagnitude of the electromagnetic voltage induced in the core section-58as compared to the magnitude of the electromagnetic voltage induced inthe core30' of Fig. 2 for the same given set of conditions is onlyone-half as great.

' In order to change the magnitude and distribution of theel-ectrostaticvoltage induced in the core 54, the core sections 56, 58 and. 60 areprovided with. ground connections 70, 72 and 74, respectively. Asillustrated the ground connections 70' and 74 are connected to theoutside endflaminations .55 of the core sections 56 and 68 respectively,which end laminations are disposed on opposite ends of the core 54. Theground connection 72 is connected nearthe mid-pointof the core section58, but this isnot essentialto the functioning of the invention.However, it has been found that good results are obtained whenthe groundconnection- 72 is connected to substantially the mid-point of the coresection 58.

Inparticular, referring to Fig. 7, there is illustrated a plurality. of:electrostatic voltage gradient curves for the core 541 when differentfactors are taken into consideration. If only the capacitance betweenadjacent laminations 55 and the capacitance established by thecapacitive couplingbetween the core 5.4and its inductively relatedwindings (not shown )'is taken into consideration, an electrostaticvoltage gradient-curve 76 1s obtained for the core 54. "On the otherhand, if in addition to these capacitancesthe resistance of the core 54is taken into consideration, a voltage gradient curve '78 is obtainedfor the core. 54; Thus by grounding the core 54 at the various pointsshown in the drawing, the maximum magnitude of the'voltageappearing onthe core 54'is only one-fourth as great as the maximum magnitude of theelectrostatic voltage appearing on the core 30- illustrated in Fig. 2.

Itisbelieved that the actual voltage curve for the core 54 liessomewhere between the curves 76 and 78 and it is illustrated by a curve88. The voltage drop across the sheets of dielectric material '62, and64, as can be seen fromsthe electrostatic voltage gradient curve 88,reduces the voltage difference between the adjacent laminations 55 ofthecore, section 60 and between the. adjacent-lamination'slSS offthe upperportion ofv the core section 58. Such an action reduces theelectrostatic voltage stress on these particular; portions of "the core54.

Referring to Fig. 8, there is illustrated another embodiment of thisinvention. In order to simplify the drawings only a magnetic core 84 isillustrated. However, in practice. windings (not shown) similar to thewindingst14 and 16 of Fig. 1 are disposed in inductive relationship withthe core 84. Likewise the core 84 is disposed in a transformer case (notshown). In this particular embodiment the magnetic core 84 having apredetermined number of laminations 85 is divided into three coresections 86, 88 and 90 of substantially equal size by means of sheets ofmaterial 92 and 94 of predetermined dielectric strength, which aresimilar to the sheets of material 62 and 64 illustrated in Fig. 6.Dielectric material 87 is disposed between each of the adjacentlaminations 85 in a conventional manner. In accordance with thisinvention, the sheets of dielectric material 92 and 94 are provided inorder to prevent the establishment of a conducting path that encloses alarge cross-sectional area of the core 84 and thus limit the magnitudeof the electromagnetic voltage which can act along any path which couldbe formed within any of the three core sections 86, 88 or 90. When thecore 84 is subdivided into N core sections of substantielly qual s e, tho tage induc e e t omagnetilly in ny c o d ci cu t nvolving he core 841w ll be reduced by the factor when compared with the voltage induced ina core not so subdivided, as illustrated in'Fig. 2.

The voltage induced electrostatically across the core 84 is inverselyproportional to the capacity between a ground point and a lamination 85most remote from the.

In particular, referring to Fig. 9, the maximum electrostatic voltageappearing across any of the core sections 86, 88 or 90 is only one-sixthas great as compared to the voltage to ground of the core section 30illustrated in Fig. 2.

Referring to Fig. 9, a curve 102 represents the electrostatic voltagedistribution on the core 84 when taking into consideration only thecapacitance between adjacent laminations and the capacitance establishedby the capacitive-coupling between the core 84 and the windings (notshown) disposed in inductive relationship therewith. However, if inaddition to these capacitances the resistance ofthe core 84 is takeninto consideration, a voltage gradient curve 104 is obtained. Again itis believed that the actual voltage distribution curve for the core 84lies somewhere between the curves 102 and 104 and this actualdistribution curve is illustrated by a curve 106. Owing to therelatively large voltage drop across the sheets'of dielectric material,92 and 94, the electrostatic voltage diiference between certain adjacentlaminations 85 is reduced. In particular, this reduction occurs in theupper portions ofthe core sections 86, 88 and 90, respectivelyl In'accordance with this invention when a plurality of core sections areproduced by the insertion of dielectric material such asthe dielectricmember 28, the probability of all the core sections breaking down andhaving high core loss is very much less than the probability for asingle.

core not so subdivided. Thus, even though there may be a breakdown theuse of multiple core sections separated in accordance with thisinvention is effective in lowering as much. Therefore, the ratio ofstrength to voltage is increased by the expedient of dividing the coreinto core sections in accordance with this invention.

Although this invention has been described with reference to two orthree core sections established by the dielectric material disposedbetween adjacent core sections, it

is to be understood that this invention can be practiced by dividing atransformer core into more than three core sections by means ofdielectric material, such as the material 28 of Fig. 1. It is also to beunderstood that the size of each core section and its relative size ascompared to the other core sections of the transformer core can bedetermined by one skilled in the transformer art.

Since certain changes may be made in the above-described apparatus, anddifierent embodiments of the invention may be made without departingfrom the spirit and scope thereof, it is intended that all mattercontained in the foregoing description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

We claim as our invention:

1. In a transformer, the combination comprising, a winding, a magneticcore having a predetermined number of laminations substantiallyinsulated from one another by dielectric material and disposed ininductive relationship to the winding, thereby subjecting the core to anelectrostatic and electromagnetic field when the transformer is subje edto an impulse voltage, a member having a preset ined dielectric strengthdisposed in the core separating it into two core sections, to preventthe occurrence of a conducting path that encloses a largecross-sectional area of the core and thus limit the electro magneticvoltage which can act along any path that may be established within thetwo core sections, and thereby minimize arcing between adjacent corelaminations, and means for grounding each of the end laminationsdisposed on opposite ends of the core, whereby the magnitude anddistribution of the electrostatic voltage induced in the core ischanged, to minimize the increase in the core loss of the transformerwhich may occur when the winding is subiected to the impulse voltage.

2. In a transformer, the combination comprising, a winding, a magneticcore having a predetermined number of laminations substantiallyinsulated from one another by dielectric material and disposed ininductive relationship to the winding, subjecting the core to anelectrostatic and electromagnetic field when the transformer issubjected to an impulse voltage, members having predetermined dielectricstrengths disposed in the core separating it into three core sections,to prevent the occurrence of a conducting path that encloses a largecrss-sectional area of the core and thus limit the electromagneticvoltage which can act along any path that may be established within anyof the three core sections, and thereby minimize arcing between adjacentcore laminations, and means for grounding each of the end laminations onopposite ends of the core and for grounding the midpoint of theintermediate of the three core sections, whereby the magnitude anddistribution of the electrostatic voltage induced in the core ischanged, to minimize the increase in the core loss of the transformerwhen the winding is subjected to the impulse voltage.

3. in a transformer, the combination comprising, a winding, a magneticcore having a predetermined number of laminations substantiallyinsulated from one another by dielectric material and-disposed ininductive relationship to the winding, subjecting the core to anelectrostatic and electromagnetic field when the transformer issubjected to an impulse voltage, members havir" predetermined dielectricstrengths disposed in the core separating it into three core sections toprevent the occurrence of a conducting path that encloses a largecrosssectional area of the core and thus limit the electromagneticvoltage which can act along any path that may be established within anyof the three core sections, and thereby minimize arcing between adjacentcore laminations, the intermediate core section being of a larger sizethan the two end core sections, and means for grounding each of the endlaminations disposed on opposite ends of the core and for grounding themid-point of the intermediate of the three core sections, whereby themagnitude and distribution of the electrostatic voltage induced in thecore is changed, to minimize the increase in the core loss of thetransformer when the winding is subjected to the impulse voltage.

4. In inductive apparatus, in combination, a winding, a magnetic corehaving a predetermined number of laminations substantially insulatedfrom one another by dielectric material and disposed in inductiverelationship to the winding, subjecting the magnetic core to anelectrostatic and electromagnetic field when the inductive apparatus issubjected to an impulse voltage, means having a predetermined dielectricstrength disposed in the magnetic core separating it into a plurality ofcore sections to prevent the occurrence of a conducting path thatencloses a large cross-sectional area of the magnetic core and thuslimit the electromagnetic voltage which can act along any path which maybe established within the plurality of core sections, the size and thusthe number of the core sections depending on the magnitude of theelectromagnetic field to be met and the size of each core section beingpredetermined to resist arcing between adjacent core laminations, andmeans for grounding each of the end laminations disposed on oppositeends of the magnetic core, whereby the magnitude and distribution of theelectrostatic voltage induced in the magnetic core is changed, tominimize the increase in the core loss of the inductive apparatus whichoccurs when the winding is sub jected to the impulse voltage.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Japanese publicationNo. 18,505 to C. Okawa; lished November25, 1939.

pub-

